It's the Midsomer Malham Orchid Society's Annual Show, and the competition is fierce. Bitterly disappointed at coming in third, Henry Plummer consoles himself in secret with his real prize - a priceless, one-of-a-kind yellow Roth orchid, illegally smuggled out of Borneo. Then a classics scholar is poisoned, and her journal, written in Latin and detailing her rather naughty activities, provides Barnaby's first clue. Guest stars include Harriet Walter (Succession; Ted Lasso).